Zero code suppression
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Zero code suppression (ZCS) is a technique that used to be implemented on T1 systems to ensure ones density.
Ones density is a requirement imposed by the used of AMI as the line code on a T1. In this line code, a long series of the zeroes (i.e., more than 15) will lead to loss of clock and loss of synchronization. This was not a problem for T1 used in the PSTN because the PAM/PCM voice digitizing scheme never generated the all-zero code (i.e., 00000000). When these same facilities were used to send data, however, a long series of zeroes became possible.
To prevent this, the network equipment would reserve the 8th bit in each time slot of each T1 frame and set it to one (1). This ensured the minimum requirement of 12.5% ones density. However, it had side consequences. Because the subscriber now only had seven bits (instead of 8) 8,000 times per second, the communication channel operated at 56 kbit/s rather than 64 kbit/s. This had to be resolved in order to use the T1 to deploy an ISDN PRI. As demands for bandwidth continued to increase, the need to provide an unchannelized T1 also increased, and ZCS was not conducive to the use of the facility in this manner. Finally, ZCS still could result in a ones density near the 12.5% minimum, which would lead to minor clock slippage on the span.
Modern T1 facilities are provisioned using B8ZS, a far more efficient and flexible scheme for ensuring ones density.
